1. Field of the Invention
This invention relates to apparatus and a method for detecting arcing in dc electrical systems and especially those subject to cyclic disturbances such ac ripple and pulse width modulated loads.
2. Background Information
It is common to provide overload, and sometimes overcurrent, protection in dc electrical systems. Overload protection is typically provided by either a thermal element which emulates the heating of the distribution wiring and opens a contact when the bimetal reaches a certain temperature, or an electronic circuit which simulates the same thermal process. Overcurrent protection is typically provided by an instantaneous trip feature which opens the circuit breaker rapidly if the current exceeds a particular threshold, such as would be reached by a short circuit, and is implemented by a magnetic trip device or an electronic simulation. A fuse is a disposable thermal trip unit with no instantaneous capability.
In addition to overload and short circuit protection, there is developing interest in protection in dc electrical systems against arc faults. Arc faults involve a highly concentrated region of heat production, a type of xe2x80x9chot spotxe2x80x9d, that can result in insulation breakdown, production of combustion products, and the ejection of hot metal particles. It can also result from broken conductors or poor connections.
Arc faults can be series or parallel. Examples of a series arc are a broken wire where the ends are close enough to cause arcing, or a poor electrical connection. Parallel arcs occur between conductors of different potential including a conductor and ground. Arc faults occur in series with the source and series arcs are further in series with the load. Arc faults have a relatively high impedance. Thus, a series arc results in a reduction in load current and is not detected by the normal overload and overcurrent protection of conventional protection devices. Even the parallel arc, which almost always draws current in excess of normal rated current in a circuit, produces currents which can be sporadic enough to yield RMS values less than that required to produce a thermal trip, or at least delay operation. Effects of the arc voltage and line impedance often prevent the parallel arc from reaching current levels sufficient to actuate the instantaneous trip function.
For many reasons, automotive circuits will be migrating to higher voltages such as 36 or 42 volts which are disproportionately more prone to damage from arcs than the present 14 volt circuits, due principally to the arc voltage being between 12 and 30 volts. Even 28-volt circuits, common in the aerospace industry, have been shown to provide an environment that supports sustained arcing. The single most aggravating factor beyond that found in residential power systems is vibration with significant humidity and dirt sometimes being aggravating factors. In addition, the telecommunications field uses 24 volt (and may migrate to 48 volt) dc systems which are susceptible to arcing. Arcs at these voltages cannot preexist, i.e., must be xe2x80x9cdrawnxe2x80x9d by a contact being separated. If they are initially extinguished to an open circuit, they should not reoccur, in theory. But the presence of carbonization or the introduction of other contaminants dynamically, ionized gas (very short lived) and vibration, which can recontact the surfaces, can make multiple occurrences not uncommon. This is particularly true of a moving vehicle travelling through the elements.
The arcing, and particularly parallel arcing, in dc electrical systems causes noise which can be exploited to detect the phenomena. Unfortunately, there are other sources of noise in the dc systems. For instance, dc power generated by an alternator equipped with a rectifier typically has a ripple content. It is now becoming more common to use pulse width modulation to control loads in dc systems. Pulse width modulation generates steps in current that introduce spurious high frequency signals. In addition, low energy positive and negative spikes of twice nominal voltage (and current) values can be introduced into these dc systems by the turning off of inductive loads. Also, bidirectional currents can be introduced by regenerative breaking, battery charging, and an integrated starter/generator in automotive dc systems.
There is a need, therefore, for an improved apparatus and method for detecting arcing in dc electrical systems.
There is also a need for such apparatus in a method which is immune to nuisance detection in response to other noise that can be present in such dc systems.
There is an additional need for such an apparatus and method which is economical to manufacture and maintain.
These needs and others are satisfied by the invention which is directed to apparatus for detecting arcing in a dc electrical system having cyclic disturbances and comprises a sensor sensing the current in the dc system and generating a sensed dc current signal, a bandpass filter filtering the sensed dc current signal with a pass band selected for sensitivity to arcing noise in the dc electrical system to generate a filtered current signal, and processing means processing the filtered current signal. The processing means integrates the filtered current signal over repetitive intervals to generate integrated filtered current values. It then generates a time attenuated accumulation of the integrated filtered current values and produces an output signal indicating the presence of arcing when the time attenuated accumulation reaches a predetermined value. In order to be able to implement the invention utilizing an inexpensive digital processor, which has a sampling capability slower than the pass band of the bandpass.filter sensitive to arcing noise, the processing means includes a resettable analog integrator which integrates the absolute value filtered current signal. The inexpensive digital processor then samples the integrated values during each repetitive period and uses the digitized results to calculate the time attenuated accumulation.
In order to further immunize the apparatus from nuisance generation of an arcing signal, only integrated values which are generated during predetermined time periods following instances where the sensed dc current signal exceeds an average value of the sensed dc current signal in a predetermined manner, such as by a selected margin or multiple, are used to generate the time attenuated accumulation. Thus, the processing means can include a comparator which compares the sensed dc current signal to the average sensed dc current signal and enables the microprocessor to accept samples for a predetermined time period which can extend to multiple integration cycles.
As another aspect of the invention, the average sensed dc current signal which can be generated by a low pass filter can also be processed by the microprocessor to provide overload protection.
In accordance with an additional aspect of the invention, the running average of a selected number of the most recent integrated filtered current values before the present value exceeds the average current value in the predetermined manner is subtracted from the present value for use in generating the time attenuated accumulation in order to further desensitize the apparatus to the cyclic disturbances such as those caused by a pulse width modulated load.
Furthermore, the invention embraces a method of detecting arcing in dc electrical systems having cyclic disturbances which includes sensing current in the dc system and generating a sensed dc current signal, bandpass filtering the sensed dc current signal in a pass band selected for sensitivity to arcing noise in the dc electrical system to generate an absolute value current signal, repetitively integrating the absolute value bandpass filtered signal over repetitive periods, generating an average sensed dc current signal, and processing the integrated signal, the sensed dc current signal and the average sensed dc current signal to determine the presence of arcing.